1. Field of the Invention
The present invention relates to an apparatus for holding a workpiece, and more particularly to an apparatus for holding a workpiece such as a semiconductor wafer for use in a polishing apparatus which polishes the workpiece to a flat mirror finish.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnections is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Conventionally, a polishing apparatus comprises a turntable having a polishing cloth thereon, a top ring for holding a workpiece such as a semiconductor wafer, a pressing device for pressing the workpiece held by the top ring against the polishing cloth on the turntable, and a driving device for rotating the top ring about its own axis. The top ring is coupled to the pressing device and the driving device through a top ring drive shaft. When the workpiece is transferred to the top ring, it is held by the lower surface of the top ring under vacuum developed in the top ring. When the workpiece is polished, a pressurized fluid such as compressed air is supplied from the top ring to the backside surface of the workpiece, thereby pressing a surface of the workpiece to be polished against a polishing surface comprising the polishing cloth on the turntable. Therefore, the top ring drive shaft coupled to the upper portion of the top ring is provided at its upper part with a rotary joint by which the top ring communicates with an external vacuum source or an external fluid source.
In the conventional polishing apparatus, the rotary joint is integrally formed with the top ring drive shaft. To be more specific, a lateral hole communicating with a vertical hole formed in the top ring drive shaft is formed in the upper part of the top ring drive shaft. By fixing the rotary joint incorporating the sealing portion therein to the top ring drive shaft, the lateral hole is caused to communicate with the connecting portion of the rotary joint which is connected to the external fluid source. Therefore, the conventional rotary-joint structure is problematic in that making the ring drive shaft is complicated and the replacement of the rotary joint is extremely troublesome when the sealing portion is damaged or worn.
Further, in the conventional rotary joint, the contacting surface between a stationary ring and a rotating ring serves as a sealing surface, and it is necessary to seal against a vacuum, pressurized air and pressurized liquid. In the case where the top ring communicates with the vacuum source through the rotary joint, a slurry-like abrasive liquid containing abrasive particles (or grains) is occasionally sucked up which then reaches, the sealing surface of the rotary joint. In this case, the abrasive liquid enters the sealing surface between the stationary ring and the rotating ring to thereby wear the sealing surface, and hence the sealing surface becomes irregular to cause fluid to leak therefrom.
Further, in the conventional rotary joint, fluid is prevented from leaking by a high contact pressure produced by making a spring force pressing the rotating ring and the stationary ring against each other larger. Therefore, the wear of the sealing surface progresses and the temperature rises in the sealing surface occurs, which causes thermal-stress cracking in either the stationary ring or the rotating ring.